There are fields, mainly that known under the name of rapid electronics, in which there are to be processed, after having been detected and then transmitted, pulse-type signals of very short duration and also of low recurrence whose initial shape may be unknown but must nevertheless be rediscovered with a certain precision in the course of processing. This may be the case in the transmission of digital signals of around a hundred megabits having a very wide pass band of around, for example, 500 Mc/s.
In such transmissions, the detected signals are converted into optical signals by means of rapid optical emitters of the laser-diode and avalanche-photodiode type upon reception, allowing the establishment of wide-band connections which may reach about 100 MHz to 1 GHz. These optical connections are effected by optical fibers which, with respect to electrical connections, have the advantage of a galvanic insulation and a wide pass band. On the other hand, these connections by optical fibers have the drawback of subjecting the transmitted signals to an attenuation which may vary significantly in the course of the transmission. This is due to the fact that the optical connectors used are extremely sensitive to mechanical tolerances and to dust, and also to the fact that the optical fibers, which are usually of the multi-strand type, may include a variable number of broken strands which still further attenuate the transmission of light. In addition to these variations in level there are the variations produced by the emitting and receiving elements.
The emitting element is the transducer which receives from the detector the signal to be processed and converts it into an optical signal. Generally, this emitting element may be a solid laser or an electroluminescent diode which has a current/optical-power conversion rate variable with time. The optical receiver, detecting the signals transmitted through the fibers constituting the emitter-receiver optical connection, may employ two types of diodes, PIN or avalanche. The PIN diodes are stable with temperature and as a function of the biasing voltage. The avalanche photodiodes, whose sensitivity to light is about 100 times greater, are on the other hand very sensitive to variations in temperature and voltage.
This review of the drawbacks presented by the transmission systems employing optical connectors, as opposed to the advantages afforded by the use of such connections, indicates that care must be taken regarding the stability of the systems. In these systems, the emitter and receiver elements are usually stabilized by control loops. The variations in level which still exist then arise almost exclusively from the optical connections.
According to the prior art, where it is desired to know, with more or less precision depending on each case, the losses undergone by the signals in the optical connection so as to be able, by acting on the gain, to re-establish the level of the signal at the value it had at the input of the connection, there is superimposed on the signal to be processed, termed the useful signal, a pilot signal of predetermined and known level which is located outside the pass band of the useful signal. As the attenuations undergone by the pilot signal are identical to those undergone by the useful signal, the level of the useful signal may be re-established upon reception.
The pilot signal is extracted upon reception from the composite signal comprising the useful signal and the pilot signal and, after detection, controls a variable-gain device.
FIG. 1 shows, diagrammatically, such a device according to the prior art.
The useful signal, combined with the pilot signal after transmission in the optical connection considered to form an incoming signal Sc, is received at a photodiode 1 whose output is connected to an amplifier 3 comprising an attenuator. The output of this amplifier is connected, on the one hand, to the input 9 of the device (load) where the useful signal Su is collected, by way of a high-pass filter 8, and, on the other hand, to an assembly of circuits constituting a loop which is closed onto the amplifier 3. This loop comprises a band-pass filter 4 which isolates the pilot signal, a rectifier 5 without threshold which delivers the pilot signal of variable amplitude, and an operational amplifier 6 which receives the pilot signal issuing from the rectifier 5 and further receives a reference signal 7. The output signal of the amplifier 6 controls the gain of the amplifier. Photodiode 1 lies between positive supply voltage +V and ground in series with a resistor 2.
The automatic-gain-control circuitry applied to a transmission system with optical connection is realized in accordance with the prior art in an analog manner, employing PIN diodes, double-gate MOS transistors, etc. However, these analog realizations have the drawback of being limited as to pass band in the region of the low or the high frequencies, depending on the components used. They also have the drawback of producing distortions and above all of varying the frequency-response curve at the same time as the gain. Moreover, as the gain-control function is not linear, it is necessary to employ a closed loop to control the amplifier, which may present problems of stability in certain cases.